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+This is a (DC style) magic wall. It gets activated for a limited
+time by rocks or gems that fall on it. Objects falling though
+it will be changed to other objects. After it has stopped running,
+it cannot be activated again.
+
+The duration is expressed in seconds. A duration of zero will let the wall
+run forever. All regular magic walls run together; however, BD style magic
+walls have a separate counter.